Composite substrate for electro-optic element and method for manufacturing the same

ABSTRACT

A composite substrate for an electro-optic element includes: an electro-optic crystal substrate having an electro-optic effect; a support substrate bonded to the electro-optic crystal substrate at least via an amorphous layer; and a low-refractive-index layer located between the electro-optic crystal substrate and the amorphous layer and having a lower refractive index than the electro-optical crystal substrate. The amorphous layer is constituted of one or more elements that constitute a layer or a substrate contacting the amorphous layer from one side and one or more elements that constitute a layer or a substrate contacting the amorphous layer from another side.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of prior U.S. application Ser. No. 16/950,306,filed Nov. 17, 2020, which is a continuation of PCT/JP2018/019657, filedon May 22, 2018, the entire contents of which are hereby incorporated byreference.

TECHNICAL FIELD

The art disclosed herein relates to a composite substrate for anelectro-optic element (such as an optical modulator) that uses anelectro-optic effect.

BACKGROUND

An electro-optic element such as an optical modulator is known. Theelectro-optic element is configured to use an electro-optic effect toconvert an electric signal to an optical signal. Electro-optic elementsare employed in radio-over-fiber technologies, for example, anddevelopment thereof is in progress to realize fast and large-capacitycommunication.

Japanese Patent Application Publication No. 2010-85789 describes anoptical modulator. This optical modulator is configured by using acomposite substrate. The composite substrate includes an electro-opticcrystal substrate having an electro-optic effect and a support substratebonded to the electro-optic crystal substrate.

SUMMARY

In a conventional composite substrate, an electro-optic crystalsubstrate and a support substrate are bonded by an adhesive. With such aconfiguration, the adhesive may degrade over time, as a result of which,a separation may occur in the composite substrate, and damages such as acrack may occur in the electro-optic crystal substrate due to thisseparation. To avoid such issues, the electro-optic crystal substratemay be directly bonded to the support substrate without using theadhesive. However, when the electro-optic crystal substrate and thesupport substrate are directly bonded, an amorphous layer constituted ofelements of the electro-optic crystal substrate and elements of thesupport substrate is formed between the electro-optic crystal substrateand the support substrate. This amorphous layer does not have acrystallinity, its optical property differs from those of the substrateson its both sides, and an interface between the electro-optic crystalsubstrate and the amorphous layer is not flat. Such a non-flat interfacehas a risk of diffusing (such as diffused reflection or leakage) andabsorbing light transmitted through the electro-optic crystal substrate.

Thus, the disclosure herein provides a composite substrate which mayavoid or reduce the aforementioned issues, and a method of manufacturingthe same.

The disclosure herein discloses a composite substrate for anelectro-optic element. This composite substrate comprises: anelectro-optic crystal substrate having an electro-optic effect; asupport substrate bonded to the electro-optic crystal substrate at leastvia an amorphous layer; and a low-refractive-index layer located betweenthe electro-optic crystal substrate and the amorphous layer and having alower refractive index than the electro-optical crystal substrate. Theamorphous layer is constituted of one or more elements that constitute alayer or a substrate contacting the amorphous layer from one side andone or more elements that constitute a layer or a substrate contactingthe amorphous layer from another side.

The aforementioned composite substrate can be manufactured by amanufacturing method as follows. This manufacturing method comprises:depositing at least one layer comprising a low-refractive-index layer onan electro-optic crystal substrate having an electro-optic effect, thelow-refractive-index layer having a lower refractive index than theelectro-optic crystal substrate; and directly bonding the electro-opticcrystal substrate on which the at least one layer is deposited to asurface of a support substrate. The wording “directly bonding” referredherein means bonding in which atoms diffuse between two members that arebonded and a covalent bond is formed between those atoms. Prior to thedirect bonding, at least one layer may be deposited on the surface ofthe support substrate.

According to the aforementioned manufacturing method, the compositesubstrate in which the support substrate is bonded to the electro-opticcrystal substrate can be manufactured without using an adhesive. Theamorphous layer resulting from the direct bonding is formed in themanufactured composite substrate, however, the low-refractive-indexlayer is interposed between the amorphous layer and the electro-opticcrystal substrate, as a result of which, the amorphous layer is not incontact with the electro-optic crystal substrate. As such, lighttransmitted through the electro-optic crystal substrate is not diffusednor absorbed by the amorphous layer or at a non-flat interface betweenthe amorphous layer and the electro-optic crystal substrate. Inaddition, the low-refractive-index layer that is in contact with theelectro-optic crystal substrate has a lower refractive index than theelectro-optic crystal substrate, thus, similar to a cladding in anoptical fiber, the low-refractive-index layer can suppress the leakageof the light transmitted through the electro-optic crystal substrate. Byusing this composite substrate, an electro-optic element with a highperformance and durability can be manufactured.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view schematically showing a composite substrate10 of a first embodiment.

FIG. 2 schematically shows a cross-sectional structure of the compositesubstrate 10 of the first embodiment.

FIG. 3 shows a step of a method of manufacturing the composite substrate10 of the first embodiment.

FIG. 4 shows a step of the method of manufacturing the compositesubstrate 10 of the first embodiment.

FIG. 5 shows a step of the method of manufacturing the compositesubstrate 10 of the first embodiment.

FIG. 6 shows a variant of the composite substrate 10, and electrodes 32,34 configured to generate an electric field in an electro-optic crystalsubstrate 12 and an optical waveguide region 36 arranged in theelectro-optic crystal substrate 12 are added.

FIG. 7 shows a variant of the composite substrate 10, and a ridgeportion 13 is arranged on a top surface 12 a of the electro-opticcrystal substrate 12.

FIG. 8 shows a variant of the composite substrate 10, and, as comparedto the variant shown in FIG. 7 , a first electrode 42 and a secondelectrode 44 are added. In this variant, a c-axis of the electro-opticcrystal substrate 12 is parallel to the electro-optic crystal substrate12.

FIG. 9 shows a variant of the composite substrate 10, and, as comparedto the variant shown in FIG. 7 , a first electrode 52 and a secondelectrode 54 are added. In this variant, the c-axis of the electro-opticcrystal substrate 12 is perpendicular to the electro-optic crystalsubstrate 12.

FIG. 10 schematically shows a cross-sectional structure of a compositesubstrate 10 a of a second embodiment.

FIG. 11 is a diagram explaining a method of manufacturing the compositesubstrate 10 a of the second embodiment.

FIG. 12 schematically shows a cross-sectional structure of a compositesubstrate 10 b of a third embodiment.

FIG. 13 shows a diagram explaining a method of manufacturing thecomposite substrate 10 b of the third embodiment.

FIG. 14 schematically shows a cross-sectional structure of a compositesubstrate 10 c of a fourth embodiment.

FIG. 15 shows a diagram explaining a method of manufacturing thecomposite substrate 10 c of the fourth embodiment.

FIG. 16 schematically shows a cross-sectional structure of a compositesubstrate 10 d of a fifth embodiment.

FIG. 17 shows a diagram explaining a method of manufacturing thecomposite substrate 10 d of the fifth embodiment.

FIG. 18 schematically shows a cross-sectional structure of a compositesubstrate 10 e of a sixth embodiment.

FIG. 19 shows a diagram explaining a method of manufacturing thecomposite substrate 10 e of the sixth embodiment.

FIG. 20 schematically shows a cross-sectional structure of a compositesubstrate 10 f of a seventh embodiment.

FIG. 21 shows a diagram explaining a method of manufacturing thecomposite substrate 10 f of the seventh embodiment.

FIG. 22 schematically shows a cross-sectional structure of a compositesubstrate 10 g of an eighth embodiment.

FIG. 23 shows a diagram explaining a method of manufacturing thecomposite substrate 10 g of the eighth embodiment.

FIG. 24 schematically shows a cross-sectional structure of a compositesubstrate 10 h of a ninth embodiment.

FIG. 25 shows a diagram explaining a method of manufacturing thecomposite substrate 10 h of the ninth embodiment.

FIG. 26 schematically shows a cross-sectional structure of a compositesubstrate 10 i of a tenth embodiment.

FIG. 27 shows a diagram explaining a method of manufacturing thecomposite substrate 10 i of the tenth embodiment.

FIG. 28 schematically shows a cross-sectional structure of a compositesubstrate 10 j of an eleventh embodiment.

FIG. 29 shows a diagram explaining a method of manufacturing thecomposite substrate 10 j of the eleventh embodiment.

DETAILED DESCRIPTION

In an embodiment of the disclosure herein, an electro-optic crystalsubstrate may be a substrate of lithium niobate (LiNbO₃:LN), lithiumtantalate (LiTaO₃:LT), potassium titanyl phosphate (KTiOPO₄:KTP),potassium lithium niobate (K_(x)Li_((1-x))NbO₂:KLN), potassium niobate(KNbO₃:KN), potassium tantalate niobate (KNb_(x)Ta_((1-x))O₃:KTN), or asolid solution of lithium niobate and lithium tantalite.

In an embodiment of the disclosure herein, the low-refractive-indexlayer may be constituted of at least one selected from a groupconsisting of silicon oxide (SiO₂), tantalum oxide (Ta₂O₅), aluminumoxide (Al₂O₃), magnesium fluoride (MgF₂), and calcium fluoride (CaF₂).

In an embodiment of the disclosure herein, the support substrate may bea substrate of silicon (Si), glass, sialon (Si₃N₄—Al₂O₃), mullite(3Al₂O₃.2SiO₂, 2Al₂O₃.SiO₂), aluminum nitride (AlN), silicon nitride(Si₃N₄), magnesium oxide (MgO), sapphire, quartz, crystal, galliumnitride (GaN), silicon carbide (SiC), or gallium oxide (Ga₂O₃).

In an embodiment of the disclosure herein, the layer or substratecontacting the amorphous layer from the one side may be thelow-refractive-index layer. Such a composite substrate can bemanufactured by directly bonding the electro-optic crystal substrate onwhich the low-refractive-index layer is deposited to the supportsubstrate. In this configuration, at least one layer may be deposited onthe surface of the support substrate in advance.

In an embodiment of the art disclosed herein, the composite substratemay further comprise a first conductive layer located between thelow-refractive-index layer and the amorphous layer. In this case, thelayer or substrate contacting the amorphous layer from the one side maybe the first conductive layer. Such a composite substrate can bemanufactured by directly bonding the electro-optic crystal substrate onwhich the first conductive layer is deposited on thelow-refractive-index layer to the support substrate. In thisconfiguration, at least one layer may be deposited on the surface of thesupport substrate in advance. In the electro-optic element manufacturedfrom the composite substrate, the first conductive layer may be used asan electrode for applying an electric signal (that is, an electricfield) to the electro-optic crystal substrate. Alternatively, the firstconductive layer may be used as a shield for suppressing the electricfield from leaking from the electro-optic element.

The first conductive layer may comprise a layer of gold (Au), silver(Ag), copper (Cu), aluminum (Al), platinum (Pt), or an alloy containingat least two of the aforementioned elements. The first conductive layermay have a single layer structure or a multiple layer structure.

A surface layer of the first conductive layer contacting the amorphouslayer may be constituted of platinum. Platinum is a material suitablefor direct bonding. Due to this, when the surface layer of the firstconductive layer is constituted of platinum, the electro-optic crystalsubstrate on which the first conductive layer is deposited can suitablybe bonded directly to the support substrate.

In the aforementioned embodiment, the composite substrate may furthercomprise a first bonding layer located between the low-refractive-indexlayer and the amorphous layer. In this case, the layer or substratecontacting the amorphous layer from the one side may be the firstbonding layer. A material constituting the first bonding layer may be,but not particularly limited to, a material suitable for direct bonding,such as tantalum oxide (Ta₂O₅), niobium oxide (Nb₂O₅), silicon (Si),aluminum oxide (Al₂O₃), and titanium oxide (TiO₂), for example.

In the aforementioned embodiment, the composite substrate may furthercomprise a first conductive layer located between thelow-refractive-index layer and the first bonding layer. In theelectro-optic element manufactured from the composite substrate, thefirst conductive layer may be used as an electrode for applying anelectric signal (that is, an electric field) to the electro-opticcrystal substrate. Alternatively, the first conductive layer may be usedas a shield for suppressing the electric field from leaking from theelectro-optic element.

In an embodiment of the disclosure herein, the layer or substratecontacting the amorphous layer from the other side may be the supportsubstrate. Such a composite substrate can be manufactured by directlybonding the electro-optic crystal substrate to the surface of thesupport substrate. In this configuration, the first conductive layerand/or the first bonding layer may be arranged in addition to thelow-refractive-index layer on the electro-optic crystal substrate.

In an embodiment of the disclosure herein, the composite substrate mayfurther comprise a second bonding layer located between the amorphouslayer and the support substrate. In this case, the layer or substratecontacting the amorphous layer from the other side may be the secondbonding layer. A material constituting the second bonding layer may be,but not particularly limited to, a material suitable for direct bonding,such as tantalum oxide (Ta₂O₅), niobium oxide (Nb₂O₅), silicon (Si),aluminum oxide (Al₂O₃), and titanium oxide (TiO₂), for example.

In the aforementioned embodiment, the composite substrate may furthercomprise a second low-refractive-index layer located between the secondbonding layer and the support substrate and having a lower refractiveindex than the electro-optic crystal substrate. Generally, therefractive index is proportional to a square root of the dielectricconstant, thus the second low-refractive-index layer has a lowdielectric constant. As such, when the composite substrate has thesecond low-refractive-index layer, it becomes easy to satisfy a speedmatching condition and adjust a characteristic impedance in theelectro-optic element manufactured from the composite substrate.Further, since stray capacitance and dielectric losses can be reduced,fast-speed operation and voltage reduction of the electro-optic elementare enabled.

In the aforementioned embodiment, the composite substrate may furthercomprise a second conductive layer located between the secondlow-refractive-index layer and the support substrate. In theelectro-optic element manufactured from the composite substrate, thesecond conductive layer may be used as an electrode for applying anelectric signal (that is, an electric field) to the electro-opticcrystal substrate. Alternatively, the second conductive layer may beused as a shield for suppressing the electric field from leaking fromthe electro-optic element.

In an embodiment of the disclosure herein, the composite substrate mayfurther comprise a second conductive layer located between the amorphouslayer and the support substrate. In this case, the layer or substratecontacting the amorphous layer from the other side may be the secondconductive layer. Such a composite substrate can be manufactured bydirectly bonding the electro-optic crystal substrate on which the secondconductive layer is deposited to the support substrate. In theelectro-optic element manufactured from the composite substrate, thesecond conductive layer may be used as an electrode for applying anelectric signal (that is, an electric field) to the electro-opticcrystal substrate. Alternatively, the second conductive layer may beused as a shield for suppressing the electric field from leaking fromthe electro-optic element.

The second conductive layer may comprise a layer of gold (Au), silver(Ag), copper (Cu), aluminum (Al), platinum (Pt), or an alloy containingat least two of the aforementioned elements. The second conductive layermay have a single layer structure or a multiple layer structure.

A surface layer of the second conductive layer contacting the amorphouslayer may be constituted of platinum. Platinum is a material suitablefor direct bonding. Due to this, when the surface layer of the secondconductive layer is constituted of platinum, the support substrate onwhich the second conductive layer is deposited can suitably be bondeddirectly to the electro-optic crystal substrate.

In an embodiment of the disclosure herein, the composite substrate mayfurther comprise a second low-refractive-index layer located between theamorphous layer and the support substrate and having a lower refractiveindex than the electro-optic crystal substrate. In this case, the layeror substrate contacting the amorphous layer from the other side may bethe second low-refractive-index layer. As aforementioned, since therefractive index is proportional to the square root of the dielectricconstant, the second low-refractive-index layer has a low dielectricconstant. As such, when the composite substrate has the secondlow-refractive-index layer, the dielectric loss is reduced in theelectro-optic element manufactured from the composite substrate.

In the aforementioned embodiment, the composite substrate may furthercomprise a second conductive layer located between the secondlow-refractive-index layer and the support substrate. In theelectro-optic element manufactured from the composite substrate, thesecond conductive layer may be used as an electrode for applying anelectric signal (that is, an electric field) to the electro-opticcrystal substrate. Alternatively, the second conductive layer may beused as a shield for suppressing the electric field from leaking fromthe electro-optic element.

In an embodiment of the disclosure herein, a ridge portion may bedefined on a surface of the electro-optic crystal substrate. When theridge portion is defined in advance on the composite substrate, theelectro-optic element requiring a ridge type light waveguide can easilybe manufactured. In addition to or as an alternative to the ridgeportion, the electro-optic crystal substrate may comprise an opticalwaveguide region doped with impurities (such as titanium or zinc). Theoptical waveguide region doped with impurities exhibits a small increasein the refractive index due to the doped impurities, by which an opticalconfinement effect thereof is small, and an optical near field pattern(near field diameter) becomes relatively large. As a result, an electricfield efficiency decreases in the electro-optic element manufacturedfrom the composite substrate, and a required driving voltage becomeslarger. Due to this, an element size also becomes larger. From aspectsof driving voltage reduction and size reduction, a ridge type opticalwaveguide with a large optical confinement effect is preferable.

In the aforementioned embodiment, a c-axis (that is, a crystallographicaxis) of the electro-optic crystal substrate may be parallel to theelectro-optical crystal substrate. That is, the electro-optic crystalsubstrate may be an x-cut or y-cut substrate. In this case, thecomposite substrate may further comprise a first electrode located onone side surface of the ridge portion and a second electrode located onanother side surface of the ridge portion and opposing the firstelectrode across the ridge portion. These first and second electrodesmay be used as electrodes for applying an electric signal (that is, anelectric field) to the ridge type optical waveguide upon manufacturingthe electro-optic element from the composite substrate.

Alternatively, the c-axis (that is, the crystallographic axis) of theelectro-optic crystal substrate may be perpendicular to theelectro-optic crystal substrate. That is, the electro-optic crystalsubstrate may be a z-cut substrate. In this case, the compositesubstrate may further comprise a first electrode located on a topsurface of the ridge portion and a second electrode located in a regionwithin the surface of the electro-optic crystal substrate and excludingthe ridge portion. These first and second electrodes may be used aselectrodes for applying an electric signal (that is, an electric field)to the ridge type optical waveguide in the electro-optic elementmanufactured from the composite substrate.

In an embodiment in which the electro-optic crystal substrate comprisesthe ridge portion, an optical waveguide region containing impurities maybe defined within the ridge portion along a longitudinal direction ofthe ridge portion. According to such a configuration, by changing aregion where the impurities are to be doped, a desired optical waveguidecan easily be fabricated without modifying the ridge portion.

Representative, non-limiting examples of the present disclosure will nowbe described in further detail with reference to the attached drawings.This detailed description is merely intended to teach a person of skillin the art further details for practicing aspects of the presentteachings and is not intended to limit the scope of the presentdisclosure. Furthermore, each of the additional features and teachingsdisclosed below may be utilized separately or in conjunction with otherfeatures and teachings to provide improved composite substrates, as wellas methods for using and manufacturing the same.

Moreover, combinations of features and steps disclosed in the followingdetailed description may not be necessary to practice the presentdisclosure in the broadest sense and are instead taught merely toparticularly describe representative examples of the present disclosure.Furthermore, various features of the above-described and below-describedrepresentative examples, as well as the various independent anddependent claims, may be combined in ways that are not specifically andexplicitly enumerated in order to provide additional useful embodimentsof the present teachings.

All features disclosed in the description and/or the claims are intendedto be disclosed separately and independently from each other for thepurpose of the original written disclosure, as well as for the purposeof restricting the claimed subject matter, independent of thecompositions of the features in the embodiments and/or the claims. Inaddition, all value ranges or indications of groups of entities areintended to disclose every possible intermediate value or intermediateentity for the purpose of the original written disclosure, as well asfor the purpose of restricting the claimed subject matter.

EMBODIMENTS

(First Embodiment) A composite substrate 10 and a method ofmanufacturing the same will be described with reference to the drawings.The composite substrate 10 of the present embodiment may be employed invarious electro-optic elements such as an optical modulator. As shown inFIG. 1 , the composite substrate 10 of the present embodiment ismanufactured in a form of a so-called wafer and is supplied to amanufacturer of the electro-optic element as such. Although this ismerely an example, the diameter of the composite substrate 10 is about10 centimeters (4 inches). Normally, a plurality of electro-opticelements is manufactured from one composite substrate 10. The compositesubstrate 10 may not be limited to the form of a wafer and may bemanufactured in various forms and supplied as such.

As shown in FIGS. 1 and 2 , the composite substrate 10 includes anelectro-optic crystal substrate 12, a support substrate 14, alow-refractive-index layer 16, and an amorphous layer 18. Theelectro-optic crystal substrate 12 is bonded to the support substrate 14via the low-refractive-index layer 16 and the amorphous layer 18. Thesesubstrates 12, 14 and layers 16, 18 extend parallel to each otherthroughout the composite substrate 10.

The electro-optic crystal substrate 12 includes a top surface 12 aexposed to the outside and a bottom surface 12 b located inside thecomposite substrate 10. A part of or an entirety of the electro-opticcrystal substrate 12 is structured as an optical waveguide configured totransmit light in an electro-optic element manufactured from thecomposite substrate 10. The electro-optic crystal substrate 12 isconstituted of a crystal of a material having an electro-optic effect.More specifically, when an electric field is applied to theelectro-optic crystal substrate 12, the refractive index of theelectro-optic crystal substrate 12 thereby changes. Specially, when theelectric field is applied along a c-axis of the electro-optic crystalsubstrate 12, the refractive index of the electro-optic crystalsubstrate 12 changes significantly. Here, the c-axis of theelectro-optic crystal substrate 12 may be parallel to the electro-opticcrystal substrate 12. That is, the electro-optic crystal substrate 12may for example be a x-cut or y-cut substrate. Alternatively, the c-axisof the electro-optic crystal substrate 12 may be perpendicular to theelectro-optic crystal substrate 12. That is, the electro-optic crystalsubstrate 12 may for example be a z-cut substrate. A thickness T12 ofthe electro-optic crystal substrate 12 may for example be, but notparticularly limited to, 0.1 micrometers or more and 10 micrometers orless.

The material constituting the electro-optic crystal substrate 12 may be,but not particularly limited to, a substrate of lithium niobate(LiNbO₃:LN), lithium tantalate (LiTaO₃:LT), potassium titanyl phosphate(KTiOPO₄:KTP), potassium lithium niobate (K_(x)Li_((1-x))NbO₂:KLN),potassium niobate (KNbO₃:KN), potassium tantalate niobate(KNb_(x)Ta_((1-x))O₃:KTN), or a solid solution of lithium niobate andlithium tantalite. The electro-optic crystal substrate 12 may have anelectro-optic effect of changing other optical constants, in addition toor as an alternative to the refractive index.

The support substrate 14 includes a top surface 14 a located inside thecomposite substrate 10 and a bottom surface 14 b exposed to the outside.The electro-optic crystal substrate 12 is bonded to the supportsubstrate 14. The support substrate 14 is provided to increase thestrength of the composite substrate 10, by which the thickness of theelectro-optic crystal substrate 12 can be reduced. The support substrate14 may be, but not particularly limited to, a substrate of silicon (Si),glass, sialon (Si₃N₄—Al₂O₃), mullite (3Al₂O₃.2SiO₂, 2Al₂O₃.SiO₂),aluminum nitride (AlN), silicon nitride (Si₃N₄), magnesium oxide (MgO),sapphire, quartz, crystal, gallium nitride (GaN), silicon carbide (SiC),or gallium oxide (Ga₂O₃), for example. In order to suppress the thermaldeformation (especially warping) of the composite substrate 10, thelinear expansion coefficient of a material constituting the supportsubstrate 14 is preferably closer to the linear expansion coefficient ofthe material constituting the electro-optic crystal substrate 12.Although not particularly limited, the linear expansion coefficient of amaterial constituting the support substrate 24 is preferably within arange of ±50 percent of the linear expansion coefficient of the materialconstituting the electro-optic crystal substrate 12.

The low-refractive-index layer 16 is located between the electro-opticcrystal substrate 12 and the amorphous layer 18 and is disposed alongthe bottom surface 12 b of the electro-optic crystal substrate 12. Thelow-refractive-index layer 16 has a lower refractive index than theelectro-optic crystal substrate 12. Due to this, the light transmittedthrough the electro-optic crystal substrate 12 tends to be fullyreflected on the bottom surface 12 b of the electro-optic crystalsubstrate 12 (that is, an interface contacting the low-refractive-indexlayer 16) and thus is suppressed from leaking from the electro-opticcrystal substrate 12. The low-refractive-index layer 16 may beconstituted of, but not particularly limited to, at least one selectedfrom a group consisting of silicon oxide (SiO₂), tantalum oxide (Ta₂O₅),aluminum oxide (Al₂O₃), magnesium fluoride (MgF₂), and calcium fluoride(MgF₂), for example. A thickness T16 of the low-refractive-index layer16 may be, but not particularly limited to, 0.1 micrometer or more and10 micrometers or less, for example.

Generally, a refractive index is proportional to the square root of thedielectric constant. Since the refractive index of thelow-refractive-index layer 16 is lower than the refractive index of theelectro-optic crystal substrate 12, the dielectric constant of thelow-refractive-index layer 16 is lower than the dielectric constant ofthe electro-optic crystal substrate 12. Thus, when the compositesubstrate 10 has the low-refractive-index layer 16, it becomes easy tosatisfy a speed matching condition and adjust a characteristic impedancein the electro-optic elements manufactured from the composite substrate10. Further, since stray capacitance and dielectric loss can be reduced,fast-speed operation and voltage reduction of the electro-optic elementare enabled.

The amorphous layer 18 is located between the low-refractive-index layer16 and the support substrate 14. The amorphous layer 18 has an amorphousstructure and is constituted of elements constituting thelow-refractive-index layer 16 contacting the amorphous layer 18 fromabove and elements constituting the support substrate 14 contacting theamorphous layer 18 from below. The thickness T18 of the amorphous layer18 may be, but not particularly limited to, 0.1 nanometer or more and100 nanometers or less. As will be described later, the compositesubstrate 10 can be manufactured by directly bonding the supportsubstrate 14 to the electro-optic crystal substrate 12 on which thelow-refractive-index layer 16 is deposited. The amorphous layer 18 is alayer generated by this direct bonding and is formed by atoms in thelow-refractive-index layer 16 and the support substrate 14 diffusing. Assuch, a top surface 18 a of the amorphous layer 18 (that is, aninterface contacting the low-refractive-index layer 16) and a bottomsurface 18 b of the amorphous layer 18 (that is, an interface contactingthe support substrate 14) are not necessarily flat.

Generally, an amorphous layer generated by the direct bonding isconstituted of elements constituting materials located above and belowthe amorphous layer and does not have crystallinity, and differentelements may be incorporated to the amorphous layer from outside, thusthe optical properties of the amorphous layer differs from those of thematerials located above and below. Further, the interfaces of theamorphous layer are not flat, and optical absorption and diffusion mayoccur. Due to this, if the amorphous layer 18 directly contacts theelectro-optic crystal substrate 12, the light transmitted through theelectro-optic crystal substrate 12 is attenuated by the amorphous layer18. Contrary to this, in the composite substrate 10 of the presentembodiment, the low-refractive-index layer 16 is interposed between theamorphous layer 18 and the electro-optic crystal substrate 12, and theamorphous layer 18 is not in contact with the electro-optic crystalsubstrate 12. As such, the light transmitted through the electro-opticcrystal substrate 12 is not diffused by the amorphous layer 18 or on thetop surface 18 a thereof. In addition, the low-refractive-index layer 16contacting the electro-optic crystal substrate 12 has the lowerrefractive index than the electro-optic crystal substrate 12, by whichit can suppress the leakage of the light transmitting through theelectro-optic crystal substrate 12 and propagate the same in the opticalwaveguide, similar to a clad in an optical fiber.

As above, in the composite substrate 10 of the present embodiment, sincethe electro-optic crystal substrate 12 is strengthened by the supportsubstrate 14, the thickness T12 of the electro-optic crystal substrate12 can be made relatively small. Further, since an optical waveguidestructure with an excellent optical confinement effect can beconstructed due to the low-refractive-index layer 16, the light can bepropagated in a high-quality bulk crystal with no impurities dopedtherein. Further, the electro-optic crystal substrate 12 and the supportsubstrate 14 are directly bonded without using an adhesive and thus arefree of deterioration and deformation of the adhesive and have a highreliability. Further, a dielectric loss caused by the adhesive also doesnot occur. Further, since the amorphous layer 18 generated by the directbonding is separated from the electro-optic crystal substrate 12 by thelow-refractive-index layer 16, the light transmitted through theelectro-optic crystal substrate 12 can be propagated toward an outputside without causing any loss.

Next, a method of manufacturing the composite substrate 10 will bedescribed with reference to FIGS. 3 to 5 . Firstly, as shown in FIG. 3 ,the electro-optic crystal substrate 12 is prepared. The electro-opticcrystal substrate 12 may be an x-cut or y-cut substrate (with the c-axisbeing parallel to the substrate). Further, in a case where a reversepolarized portion is formed, the electro-optic crystal substrate 12 maybe an offset substrate in which the c-axis and a horizontal surface ofthe substrate meet at an angle of 10° or less. Alternatively, it may bea z-cut substrate (with the c-axis being perpendicular to thesubstrate). Next, as shown in FIG. 4 , the low-refractive-index layer 16is deposited on the bottom surface 12 b of the electro-optic crystalsubstrate 12. This deposition of the low-refractive-index layer 16 maybe executed by, but not particularly limited to, vapor deposition(physical vapor deposition or chemical vapor deposition). The bottomsurface 12 b of the electro-optic crystal substrate 12 is one of themain surfaces of the electro-optic crystal substrate 12. Next, as shownin FIG. 5 , the support substrate 14 is prepared, and the supportsubstrate 14 is directly bonded to the bottom surface 12 b of theelectro-optic crystal substrate 12 on which the low-refractive-indexlayer 16 is deposited. At this occasion, the aforementioned amorphouslayer 18 is formed between the support substrate 14 and thelow-refractive-index layer 16. Due to this, the composite substrate 10shown in FIGS. 1 and 2 is manufactured.

In regard to the aforementioned direct bonding, the specific order ofprocesses and processing condition are not particularly limited. Thesemay suitably be set according to respective materials of the layers orsubstrates to be bonded to each other. As one example, in themanufacturing method of the present embodiment, firstly, aneutralization beam is irradiated onto each of the bonding surfaces in ahigh vacuum chamber (such as at about 1×10⁻⁶ Pascal). Due to this, therespective bonding surfaces are activated. Then, the activated bondingsurfaces are brought into contact with each other in a high vacuumatmosphere and are bonded at a normal temperature. A load at thisbonding can be set for example to 100 to 20000 Newtons. In thismanufacturing method, upon executing surface activation by theneutralization beam, an inert gas is introduced into the chamber and ahigh voltage is applied from a DC power source to an electrode disposedinside the chamber. Due to this, an electric field is generated betweenthe electrode (positive electrode) and the chamber (negative electrode)and then electrons move, by which a beam constituted of atoms and ionsis generated from the inert gas. When the beam reaches a grid, the ionbeam is neutralized by the grid, by which a neutral atom beam is emittedfrom a fast atom beam source. The atomic specie constituting the beam ispreferably an inert gas element (such as argon (Ar), nitrogen (N),etc.). The voltage upon the activation by the beam irradiation may beset to 0.5 to 2.0 kilovolts and the current thereof may be set to 50 to200 milliamperes.

As shown in FIG. 6 , in the composite substrate 10, electrodes 32, 34for forming an electric field in the electro-optic crystal substrate 12may be mounted on the top surface 12 a of the electro-optic crystalsubstrate 12. The material constituting the electrodes 32, 34 may simplyneed to be a conductor, and may be a metal such as gold (Au), silver(Ag), copper (Cu), aluminum (Al), and platinum (Pt). The electrodes 32,34 may each include a layer of titanium (Ti), chromium (Cr), nickel(Ni), or platinum (Pt) as a base layer (lowermost layer) contacting theelectro-optic crystal substrate 12 for preventing exfoliation andmigration of the electrodes 32, 34. The number, position and shape ofthe electrodes 32, 34 are not particularly limited. For example, as tothe number of the electrodes 32, 34, they may suitably be set accordingto the number of electro-optic elements to be manufactured from thecomposite substrate 10 and the number of the electrodes 32, 34 whicheach electro-optic element requires. When the electrodes 32, 34 arepre-mounted on the composite substrate 10, a manufacturer of theelectro-optic elements can easily manufacture the electro-optic elementsfrom the composite substrate 10. The electrodes 32, 34 shown in FIG. 6may be mounted similarly to the composite substrates 10 a to 10 j ofother embodiments to be described later.

In addition to or alternatively, an optical waveguide region 36 may bedisposed in the electro-optic crystal substrate 12 by doping impurities.In the electro-optic crystal substrate 12, the refractive index can beselectively (that is, locally) increased by doping specific impuritiessuch as titanium or zinc and the optical waveguide region 36 can therebybe constructed. The number, position and shape of the optical waveguideregion 36 are not particularly limited. For example, as to the number ofthe optical waveguide region 36, it may suitably be set according to thenumber of electro-optic elements to be manufactured from the compositesubstrate 10 and the number of the optical waveguide region 36 whicheach electro-optic element requires. When the optical waveguide region36 is disposed in advance in the composite substrate 10, themanufacturer of the electro-optic elements can easily manufacture theelectro-optic elements from the composite substrate 10. The opticalwaveguide region 36 shown in FIG. 6 may be disposed similarly in thecomposite substrates 10 a to 10 j of other embodiments to be describedlater.

As shown in FIG. 7 , a ridge portion 13 may be disposed on the topsurface 12 a of the electro-optic crystal substrate 12. The ridgeportion 13 is a protruding portion extending in an elongate shape alongthe top surface 12 a. The ridge portion 13 configures a ridge typeoptical waveguide in the electro-optic element manufactured from thecomposite substrate 10. When the ridge portion 13 is disposed on thecomposite substrate 10 in advance, electro-optic elements requiring theridge type optical waveguide can easily be manufactured. The width W ofthe ridge portion 13 may be, but not particularly limited to, 1micrometer or more and 10 micrometers or less. The height TR of theridge portion 13 may be, but also not particularly limited to, 10percent or more and 95 percent or less of the thickness T12 of theelectro-optic crystal substrate 12. The number, position and shape ofthe ridge portion 13 are also not particularly limited. Although this ismerely an example, when the composite substrate 10 is used formanufacturing a Mach-Zehnder electro-optic modulator, two ridge portions13, at least partially extending parallel to each other, are preferablydisposed. The ridge portion 13 shown in FIG. 7 may be disposed similarlyin the composite substrates 10 a to 10 j of other embodiments to bedescribed later.

As shown in FIG. 8 , the composite substrate 10 including the ridgeportion 13 may further include a first electrode 42 and a secondelectrode 44. Here, in the case where the c-axis of the electro-opticcrystal substrate 12 is parallel to the electro-optic crystal substrate12, the first electrode 42 is preferably mounted on one side surface 13a of the ridge portion 13. Further, the second electrode 44 ispreferably mounted on another side surface 13 b of the ridge portion 13to oppose the first electrode 42 across the ridge portion 13. Accordingto such a configuration, the first electrode 42 and the second electrode44 can apply an electric field parallel to the c-axis with respect tothe ridge portion 13 that is to be the optical waveguide in theelectro-optic element. A material constituting the first electrode 42and the second electrode 44 may simply need to be a conductor and may bea metal such as gold (Au), copper (Cu), and aluminum (Al). Theelectrodes 42, 44 may each include a layer of titanium (Ti), chromium(Cr), nickel (Ni), or platinum (Pt) as a base layer (lowermost layer)contacting the electro-optic crystal substrate 12 for preventingexfoliation and migration of the electrodes 42, 44. Further, alow-refractive-index film having a lower refractive index than theelectro-optic crystal substrate 12 may be disposed between the firstelectrode 42 and the electro-optic crystal substrate 12 and also betweenthe second electrode 44 and the electro-optic crystal substrate 12. Sucha low-refractive-index film functions as a cladding layer and cansuppress the loss in light transmitted through the ridge portion 13.Together with the ridge portion 13, the first electrode 42 and thesecond electrode 44 shown in FIG. 8 may be mounted similarly to thecomposite substrates 10 a to 10 j of the other embodiments to bedescribed later.

As shown in FIG. 9 , the c-axis of the electro-optic crystal substrate12 may be perpendicular to the electro-optic crystal substrate 12. Inthis case as well, the ridge portion 13 may be disposed on the topsurface 12 a of the electro-optic crystal substrate 12. Further, a firstelectrode 52 and a second electrode 54 may be mounted on the top surface12 a of the electro-optic crystal substrate 12. However, the firstelectrode 52 is preferably mounted on a top surface 13 c of the ridgeportion 13 and the second electrode 54 may preferably be provided in aregion within the top surface 12 a of the electro-optic crystalsubstrate 12 excluding the ridge portion 13. According to such aconfiguration, the first electrode 52 and the second electrode 54 canapply an electric field parallel to the c-axis with respect to the ridgeportion 13 that is to be the optical waveguide in the electro-opticelement. Together with the ridge portion 13, the first electrode 52 andthe second electrode 54 shown in FIG. 9 may be mounted similarly to thecomposite substrates 10 a to 10 j of other embodiments to be describedlater. In cases where the composite substrates 10 a to 10 j are providedwith conductive layers 20, 20′, the conductive layers 20, 20′ may eachbe used as a substitute to the second electrode 54. In these cases, thesecond electrode 54 may not be necessary and thus may be omitted.

(Second Embodiment) A composite substrate 10 a of a second embodimentwill be described with reference to FIGS. 10 and 11 . As shown in FIG.10 , the composite substrate 10 a of the second embodiment furtherincludes a first conductive layer 20 and thus differs from the compositesubstrate 10 of the first embodiment in this regard. The firstconductive layer 20 is located between the low-refractive-index layer 16and the amorphous layer 18 and contacts the amorphous layer 18 fromabove. As such, the amorphous layer 18 in the present embodiment isconstituted of elements constituting the first conductive layer 20 andthe elements constituting the support substrate 14. The first conductivelayer 20 may be used as an electrode for applying an electric signal(that is, an electric field) to the electro-optic crystal substrate 12in an electro-optic element manufactured from the composite substrate 10a. Alternatively, the first conductive layer 20 may be used as a shieldfor suppressing the electric field from leaking from the electro-opticelement.

A material constituting the first conductive layer 20 may simply be aconductor, and for example, it may include a layer of gold (Au), silver(Ag), copper (Cu), aluminum (Al), platinum (Pt), or an alloy containingat least two of the aforementioned elements. The first conductive layer20 may have a single layer structure or a multiple layer structure. Thefirst conductive layer 20 may include a layer of titanium (Ti), chromium(Cr), nickel (Ni), or platinum (Pt) as a base layer contacting thelow-refractive-index layer 16 for preventing exfoliation and migrationof the first conductive layer 20. The thickness T20 of the firstconductive layer may be, but not particularly limited to, 0.05micrometer or more and 5 micrometers or less.

As shown in FIG. 11 , the composite substrate 10 a of the presentembodiment can also be manufactured by directly bonding theelectro-optic crystal substrate 12 to the support substrate 14. However,in a manufacturing method of the present embodiment, thelow-refractive-index layer 16 and the first conductive layer 20 aredeposited on the bottom surface 12 b of the electro-optic crystalsubstrate 12 prior to performing the direct bonding. Thelow-refractive-index layer 16 is deposited on the bottom surface 12 b ofthe electro-optic crystal substrate 12 and the first conductive layer 20is deposited on the low-refractive-index layer 16. As such, the directbonding in this embodiment is performed between the first conductivelayer 20 and the support substrate 14. In this case, a surface layer 20a of the first conductive layer 20 may be constituted of platinum.Platinum is a material suitable for direct bonding. Due to this, whenthe surface layer 20 a of the first conductive layer 20 is constitutedof platinum, the electro-optic crystal substrate 12 on which the firstconductive layer 20 is deposited can suitably be bonded directly to thesupport substrate 14. The deposition of the first conductive layer 20may be performed, for example, by sputtering or vapor deposition(physical vapor deposition or chemical vapor deposition).

(Third Embodiment) A composite substrate 10 b of a third embodiment willbe described with reference to FIGS. 12 and 13 . As shown in FIG. 12 ,the composite substrate 10 b of the third embodiment further includes afirst bonding layer 22, and thus differs from the composite substrate 10b of the second embodiment in this regard. The first bonding layer 22 islocated between the first conductive layer 20 and the amorphous layer 18and contacts the amorphous layer 18 from above. As such, the amorphouslayer 18 in the present embodiment is constituted of elementsconstituting the first bonding layer 22 and the elements constitutingthe support substrate 14.

A material constituting the first bonding layer 22 preferably is, amaterial suitable for direct bonding, such as, but not particularlylimited to, tantalum oxide (Ta₂O₅), niobium oxide (Nb₂O₅), silicon (Si),aluminum oxide (Al₂O₃), and titanium oxide (TiO₂), for example. Athickness T22 of the first bonding layer 22 may be, but not particularlylimited to, 0.01 micrometer or more and 0.5 micrometer or less. Sincethe first bonding layer 22 is separated from the electro-optic crystalsubstrate 12 by the first conductive layer 20, it does not substantiallyaffect the characteristics of the composite substrate 10 b.

As shown in FIG. 13 , the composite substrate 10 b of the presentembodiment can also be manufactured by directly bonding theelectro-optic crystal substrate 12 to the support substrate 14. However,in a manufacturing method of the present embodiment, thelow-refractive-index layer 16, the first conductive layer 20 and thefirst bonding layer 22 are deposited on the bottom surface 12 b of theelectro-optic crystal substrate 12 prior to performing the directbonding. The low-refractive-index layer 16 is deposited on the bottomsurface 12 b of the electro-optic crystal substrate 12 and the firstconductive layer 20 is deposited on the low-refractive-index layer 16.The first bonding layer 22 is deposited on the first conductive layer20. As such, the direct bonding in this embodiment is performed betweenthe first bonding layer 22 and the support substrate 14. Since the firstbonding layer 22 is constituted of a material suitable for directbonding, the electro-optic crystal substrate 12 on which the firstbonding layer 22 is deposited can suitably be bonded directly to thesupport substrate 14. The deposition of the first bonding layer 22 maybe performed by, but not particularly limited to, sputtering or vapordeposition (physical vapor deposition or chemical vapor deposition).

(Fourth Embodiment) A composite substrate 10 c of a fourth embodimentwill be described with reference to FIGS. 14 and 15 . As shown in FIG.14 , the composite substrate 10 c of the fourth embodiment furtherincludes a second bonding layer 22′ and thus differs from the compositesubstrate 10 c of the third embodiment in this regard. The secondbonding layer 22′ is located between the amorphous layer 18 and thesupport substrate 14 and contacts the amorphous layer 18 from below. Assuch, the amorphous layer 18 in the present embodiment is constituted ofthe elements constituting the first bonding layer 22 and elementsconstituting the second bonding layer 22′.

A material constituting the second bonding layer 22′ preferably is amaterial suitable for direct bonding such as, but not particularlylimited to, tantalum oxide (Ta₂O₅), niobium oxide (Nb₂O₅), silicon (Si),aluminum oxide (Al₂O₃), and titanium oxide (TiO₂), for example. Thethickness T22′ of the second bonding layer 22′ may be, but notparticularly limited to, 0.01 micrometer or more and 0.5 micrometer orless. Since the second bonding layer 22′ is separated from theelectro-optic crystal substrate 12 by the first conductive layer 20, itdoes not substantially affect the characteristics of the compositesubstrate 10 c.

As shown in FIG. 15 , the composite substrate 10 c of the presentembodiment can also be manufactured by directly bonding theelectro-optic crystal substrate 12 to the support substrate 14. However,in the manufacturing method of the present embodiment, the secondbonding layer 22′ is deposited on the top surface 14 a of the supportsubstrate 14 prior to performing the direct bonding. Thelow-refractive-index layer 16, the first conductive layer 20 and thefirst bonding layer 22 are deposited on the bottom surface 12 b of theelectro-optic crystal substrate 12, similar to the third embodiment. Assuch, the direct bonding in this embodiment is performed between thefirst bonding layer 22 and the second bonding layer 22′. Since the firstbonding layer 22 and the second bonding layer 22′ are constituted ofmaterial(s) suitable for direct bonding, the electro-optic crystalsubstrate 12 on which the first bonding layer 22 is deposited cansuitably be bonded directly to the support substrate 14 on which thesecond bonding layer 22′ is deposited. The deposition of the secondbonding layer 22′ may be performed for example by vapor deposition(physical vapor deposition or chemical vapor deposition), similar to thefirst bonding layer 22.

(Fifth Embodiment) A composite substrate 10 d of a fifth embodiment willbe described with reference to FIGS. 16 and 17 . As shown in FIG. 16 ,the composite substrate 10 d of the fifth embodiment does not includethe first conductive layer 20 and thus differs from the compositesubstrate 10 b of the third embodiment in this regard. That is, thefirst conductive layer 20 may not be necessarily required. As shown inFIG. 17 , the composite substrate 10 d of the present embodiment canalso be manufactured by directly bonding the electro-optic crystalsubstrate 12 to the support substrate 14. In a manufacturing method ofthe present embodiment, the low-refractive-index layer 16 and the firstbonding layer 22 are deposited on the bottom surface 12 b of theelectro-optic crystal substrate 12 prior to performing the directbonding. The low-refractive-index layer 16 is deposited on the bottomsurface 12 b of the electro-optic crystal substrate 12, and the firstbonding layer 22 is deposited on the low-refractive-index layer 16. Assuch, the direct bonding in this embodiment is performed between thefirst bonding layer 22 and the support substrate 14.

(Sixth Embodiment) A composite substrate 10 e of a sixth embodiment willbe described with reference to FIGS. 18 and 19 . As shown in FIG. 18 ,the composite substrate 10 e of the sixth embodiment further includesthe second bonding layer 22′ and thus differs from the compositesubstrate 10 d of the fifth embodiment in this regard. In other words,the composite substrate 10 e of the present embodiment does not includethe first conductive layer 20 and thus differs from the compositesubstrate 10 c of the fourth embodiment in this regard. As shown in FIG.19 , the composite substrate 10 e of the present embodiment can also bemanufactured by directly bonding the electro-optic crystal substrate 12to the support substrate 14. In a manufacturing method of the presentembodiment, the low-refractive-index layer 16 and the first bondinglayer 22 are formed on the bottom surface 12 b of the electro-opticcrystal substrate 12 prior to performing the direct bonding. Further,the second bonding layer 22′ is deposited on the top surface 14 a of thesupport substrate 14. As such, the direct bonding in this embodiment isperformed between the first bonding layer 22 and the second bondinglayer 22′.

(Seventh Embodiment) A composite substrate 10 f of a seventh embodimentwill be described with reference to FIGS. 20 and 21 . As shown in FIG.20 , the composite substrate 10 f of the seventh embodiment furtherincludes a second conductive layer 20′ and thus differs from thecomposite substrate 10 of the first embodiment in this regard. Thesecond conductive layer 20′ is located between the amorphous layer 18and the support substrate 14 and contacts the amorphous layer 18 frombelow. As such, the amorphous layer 18 in the present embodiment isconstituted of elements constituting the low-refractive-index layer 16and elements constituting the second conductive layer 20′. The secondconductive layer 20′ may be used as an electrode for applying anelectric signal (that is, an electric field) to the electro-opticcrystal substrate 12 in an electro-optic element manufactured from thecomposite substrate 10 f. Alternatively, the second conductive layer 20′may be used as a shield for suppressing the electric field from leakingfrom the electro-optic element.

The material constituting the second conductive layer 20′ may simply bea conductor and, for example, may include a layer of gold (Au), silver(Ag), copper (Cu), aluminum (Al), platinum (Pt), or an alloy containingat least two of the aforementioned elements. The second conductive layer20′ may have a single layer structure or a multiple layer structure. Thesecond conductive layer 20′ may include a layer of titanium (Ti),chromium (Cr), nickel (Ni), or platinum (Pt) as a base layer contactingthe support substrate 14 for preventing exfoliation and migration of thesecond conductive layer 20′. The thickness T20′ of the second conductivelayer 20′ may be, but not particularly limited to, 0.05 micrometer ormore and 5 micrometers or less.

As shown in FIG. 21 , the composite substrate 10 f of the presentembodiment can also be manufactured by directly bonding theelectro-optic crystal substrate 12 to the support substrate 14. However,in the manufacturing method of the present embodiment, the secondconductive layer 20′ is deposited on the top surface 14 a of the supportsubstrate 14 prior to performing the direct bonding. Thelow-refractive-index layer 16 is deposited on the bottom surface 12 b ofthe electro-optic crystal substrate 12, similar to the first embodiment.As such, the direct bonding in this embodiment is performed between thelow-refractive-index layer 16 and the second conductive layer 20′. Inthis case, a surface layer 20 a′ of the second conductive layer 20′ maybe constituted of platinum. As aforementioned, platinum is a materialsuitable for direct bonding. Due to this, when the surface layer 20 a′of the second conductive layer 20′ is constituted of platinum, theelectro-optic crystal substrate 12 can suitably be bonded directly tothe support substrate 14 on which the second conductive layer 20′ isdeposited. The deposition of the second conductive layer 20′ may beperformed, for example, by sputtering or vapor deposition (physicalvapor deposition or chemical vapor deposition).

(Eighth Embodiment) A composite substrate 10 g of an eighth embodimentwill be described with reference to FIGS. 22 and 23 . As shown in FIG.22 , the composite substrate 10 g of the eighth embodiment furtherincludes a second low-refractive-index layer 16′ and thus differs fromthe composite substrate 10 f of the seventh embodiment in this regard.The second low-refractive-index layer 16′ has a lower refractive indexthan the electro-optic crystal substrate 12, similar to thelow-refractive-index layer 16. The second low-refractive-index layer 16′is located between the amorphous layer 18 and the support substrate 14and contacts the amorphous layer 18 from below. As such, the amorphouslayer 18 in the present embodiment is constituted of elementsconstituting the low-refractive-index layer 16 and elements constitutingthe second low-refractive-index layer 16′. The thickness T16′ of thesecond low-refractive-index layer 16′ may be, but not particularlylimited to, 0.1 micrometer or more and 10 micrometers or less, forexample.

Although not particularly limited, the second low-refractive-index layer16′ may be constituted of at least one selected from the groupconsisting of silicon oxide (SiO₂), tantalum oxide (Ta₂O₅), aluminumoxide (Al₂O₃), magnesium fluoride (MgF₂) and calcium fluoride (MgF₂).The material constituting the second low-refractive-index layer 16′ maybe the material constituting the low-refractive-index layer 16.Alternatively, the material constituting the second low-refractive-indexlayer 16′ may be different from the material constituting thelow-refractive-index layer 16. As aforementioned, the refractive indexis proportional to the square of the dielectric constant. Since therefractive index of the second low-refractive-index layer 16′ is lowerthan the refractive index of the electro-optic crystal substrate 12, thedielectric constant of the second low-refractive-index layer 16′ islower than the dielectric constant of the electro-optic crystalsubstrate 12. Thus, when the composite substrate 10 has the secondlow-refractive-index layer 16′, it becomes easy to satisfy the speedmatching condition and adjust the characteristic impedance in theelectro-optic elements manufactured from the composite substrate 10.Further, since the stray capacitance and dielectric loss can be reduced,the fast-speed operation and voltage reduction of the electro-opticelement are enabled.

As shown in FIG. 23 , the composite substrate 10 g of the presentembodiment can also be manufactured by directly bonding theelectro-optic crystal substrate 12 to the support substrate 14. In themanufacturing method of the present embodiment, the second conductivelayer 20′ and the second low-refractive-index layer 16′ are formed onthe top surface 14 a of the support substrate 14 prior to performing thedirect bonding. The second conductive layer 20′ is deposited on the topsurface 14 a of the support substrate 14, and the secondlow-refractive-index layer 16′ is deposited on the second conductivelayer 20′. The low-refractive-index layer 16 is deposited on the bottomsurface 12 b of the electro-optic crystal substrate 12, similar to thefirst embodiment. As such, the direct bonding in this embodiment isperformed between the low-refractive-index layer 16 and the secondlow-refractive-index layer 16′. The deposition of the secondlow-refractive-index layer 16′ may be performed by sputtering or vapordeposition (physical vapor deposition or chemical vapor deposition),similar to the low-refractive-index layer 16.

(Ninth Embodiment) A composite substrate 10 h of a ninth embodiment willbe described with reference to FIGS. 24 and 25 . As shown in FIG. 24 ,the composite substrate 10 h of the ninth embodiment further includesthe first bonding layer 22 and the second bonding layer 22′ and thusdiffers from the composite substrate 10 g of the eighth embodiment inthis regard. The first bonding layer 22 is located between thelow-refractive-index layer 16 and the amorphous layer 18 and contactsthe amorphous layer 18 from above. The second bonding layer 22′ islocated between the amorphous layer 18 and the secondlow-refractive-index layer 16′ and contacts the amorphous layer 18 frombelow. As such, the amorphous layer 18 in the present embodiment isconstituted of elements constituting the first bonding layer 22 andelements constituting the second bonding layer 22′.

As aforementioned, the first bonding layer 22 and the second bondinglayer 22′ can each be constituted of the material suitable for directbonding, such as tantalum oxide (Ta₂O₅), niobium oxide (Nb₂O₅), silicon(Si), aluminum oxide (Al₂O₃), and titanium oxide (TiO₂), for example.The first bonding layer 22 and the second bonding layer 22′ may beconstituted of the same material or may be constituted of differentmaterials. Thicknesses T22, T22′ of the first bonding layer 22 and thesecond bonding layer 22′ may each be, but not particularly limited to,0.01 micrometer or more and 0.5 micrometer or less. The thicknesses T22,T22′ of the first bonding layer 22 and the second bonding layer 22′ maybe the same or different.

As shown in FIG. 25 , the composite substrate 10 h of the presentembodiment can also be manufactured by directly bonding theelectro-optic crystal substrate 12 to the support substrate 14. In themanufacturing method of the present embodiment, the low-refractive-indexlayer 16 and the first bonding layer 22 are formed on the bottom surface12 b of the electro-optic crystal substrate 12 prior to performing thedirect bonding. The low-refractive-index layer 16 is deposited on thebottom surface 12 b of the electro-optic crystal substrate 12 and thefirst bonding layer 22 is deposited on the low-refractive-index layer16. The second conductive layer 20′, the second low-refractive-indexlayer 16′ and the second bonding layer 22′ are formed on the top surface14 a of the support substrate 14. The second conductive layer 20′ isdeposited on the top surface 14 a of the support substrate 14 and thesecond low-refractive-index layer 16′ is deposited on the secondconductive layer 20′. Then, the second bonding layer 22′ is deposited onthe second low-refractive-index layer 16′. As such, the direct bondingin this embodiment is performed between the first bonding layer 22 andthe second bonding layer 22′. The deposition of the first bonding layer22 and the second bonding layer 22′ may be performed by sputtering orvapor deposition (physical vapor deposition or chemical vapordeposition).

(Tenth Embodiment) A composite substrate 10 i of a tenth embodiment willbe described with reference to FIGS. 26 and 27 . As shown in FIG. 26 ,the composite substrate 10 i of the tenth embodiment does not includethe second conductive layer 20′ and thus differs from the compositesubstrate 10 h of the ninth embodiment in this regard. That is, thesecond conductive layer 20′ may not be necessarily required. As shown inFIG. 27 , the composite substrate 10 i of the present embodiment canalso be manufactured by directly bonding the electro-optic crystalsubstrate 12 to the support substrate 14. In the manufacturing method ofthe present embodiment, the low-refractive-index layer 16 and the firstbonding layer 22 are formed on the bottom surface 12 b of theelectro-optic crystal substrate 12 prior to performing the directbonding. The low-refractive-index layer 16 is deposited on the bottomsurface 12 b of the electro-optic crystal substrate 12 and the firstbonding layer 22 is deposited on the low-refractive-index layer 16. Thesecond low-refractive-index layer 16′ and the second bonding layer 22′are formed on the top surface 14 a of the support substrate 14. Thesecond low-refractive-index layer 16′ is deposited on the top surface 14a of the support substrate 14 and the second bonding layer 22′ isdeposited on the second low-refractive-index layer 16′. As such, thedirect bonding in this embodiment is performed between the first bondinglayer 22 and the second bonding layer 22′.

(Eleventh Embodiment) A composite substrate 10 j of an eleventhembodiment will be described with reference to FIGS. 28 and 29 . Asshown in FIG. 28 , the composite substrate 10 j of the eleventhembodiment does not include the first bonding layer 22 and the secondbonding layer 22′ and thus differs from the composite substrate 10 i ofthe tenth embodiment in this regard. That is, the first bonding layer 22and the second bonding layer 22′ may not be necessarily required. Asshown in FIG. 29 , the composite substrate 10 j of the presentembodiment can also be manufactured by directly bonding theelectro-optic crystal substrate 12 to the support substrate 14. In themanufacturing method of the present embodiment, the low-refractive-indexlayer 16 is deposited on the bottom surface 12 b of the electro-opticcrystal substrate 12 and the second low-refractive-index layer 16′ isdeposited on the top surface 14 a of the support substrate 14 prior toperforming the direct bonding. As such, the direct bonding in thisembodiment is performed between the low-refractive-index layer 16 andthe second low-refractive-index layer 16′.

REFERENCE LIST

-   10: Composite Substrate-   12: Electro-Optic Crystal Substrate-   13: Ridge Portion-   14: Support Substrate-   16: Low-Refractive-Index Layer-   16′: Second Low-Refractive-Index Layer-   18: Amorphous Layer-   20: First Conductive Layer-   20′: Second Conductive Layer-   22: First Bonding Layer-   22′: Second Bonding Layer

The invention claimed is:
 1. A composite substrate for an electro-opticelement, the composite substrate comprising: an electro-optic crystalsubstrate having an electro-optic effect; a support substrate bonded tothe electro-optic crystal substrate at least via an amorphous layer; alow-refractive-index layer located between the electro-optic crystalsubstrate and the amorphous layer, contacting the electro-optic crystalsubstrate, and having a lower refractive index than the electro-opticalcrystal substrate; a first conductive layer located between thelow-refractive-index layer and the amorphous layer; and a first bondinglayer located between the low-refractive-index layer and the amorphouslayer, wherein the amorphous layer contains one or more elements thatare present in a layer or a substrate that contacts the amorphous layerfrom one side and one or more elements that are present in a layer or asubstrate that contacts the amorphous layer from another side, the layeror substrate contacting the amorphous layer from the one side is thefirst bonding layer, and the first conductive layer comprises a layer ofgold, silver, copper, aluminum, platinum, or an alloy containing atleast two of gold, silver, copper, aluminum and platinum.
 2. Thecomposite substrate according to claim 1, wherein the first bondinglayer is constituted of at least one member selected from the groupconsisting of tantalum oxide, niobium oxide, silicon, aluminum oxide,and titanium oxide.